Wastewater, whether of domestic or industrial origin, should be treated properly before it is discharged into a natural water course. Major steps in conventional wastewater treatment plants involve a gas transfer, for example an oxygen transfer process. The activated sludge process, which is currently the most popular method of biological wastewater treatment, and aerobic digestion for sludge treatment each require aeration where oxygen in air is transferred into a liquid phase (wastewater). Other processes which involve gas transfer include ammonia stripping for removal of ammonia in wastewater where ammonia gas is transferred from a liquid phase to supplied air bubbles and eventually to the atmosphere. The efficiency of gas transfer depends in part on the area of the liquid gas interface as described by the two-film theory, wherein the interface area through which gas transfer can take place increases with decreasing gas bubble size at a constant gas flow rate. Gas transfer efficiency also depends on the degree of turbulence or agitation of wastewater since higher turbulence results in thinner liquid film and reduced resistance of transfer of gas into the bulk liquid phase.
Oxygen transfer (aeration) in wastewater treatment plants is an energy-intensive process which can consume as much as 50 to 90 percent of the net power demand for a treatment plant. Many treatment plants are replacing and upgrading older and less efficient aeration devices since installation of higher efficiency aeration devices reduces the cost of aeration.
One type of gas diffuser device known in the art generates fine air bubbles with a diameter of 2 to 4 mm by use of a porous plate made from alumina or silica material. The small gas bubbles emanating from small openings in the porous medium increase the gas-liquid interface area resulting in a relatively high gas transfer efficiency. However, this fine bubble gas diffuser is susceptible to clogging.
Another type of gas diffuser device, known in the art is a coarse bubble gas diffuser, is shown in U.S. Pat. Nos. 3,679,187 and 4,421,696. Both of these gas diffuser devices have a body directly above slot-shaped ports in a gas distribution tube. The gas bubble distribution body above the gas distribution tube provides a surface for gas bubbles to disperse to a shear edge on which gas bubbles are broken down to smaller sized bubbles and mixed with the water. The bottom of the gas bubble distribution body typically has a truncated pyramidal or conical shape which extends upwardly and outwardly from the vertical gas injection pipe. As a result of the shape of the bottom of the bubble distribution body, however, the gas bubbles leaving the ports in the gas distribution tube are not significantly disturbed. Thus, the bubbles rise vertically above the bubble distribution body forming a thick and dense bubble column, and gas transfer is hampered.
There is a need to create more turbulence for wider distribution of gas bubbles in liquid and break up of gas bubbles into smaller sizes, while at the same time minimizing clogging.